Key points of ESD protection design for HDMI's high-speed TMDS lines

To protect the high-speed TMDS lines of HDMI, good ESD protection design is essential. Due to the high speed of HDMI 1.3 and the increasing number of devices using multiple HDMI interfaces, some new requirements are put forward for ESD protection solutions. First, low-capacitance ESD protection is critical to maintain data integrity under high-speed conditions. Secondly, multiple HDMI ports limit the space selection of components and wiring, and their mutual impact must also be considered, which requires the package size of the protection component to be small. Finally, price is also an important consideration.

In order to meet the requirements of effective ESD protection without affecting high-speed signal transmission, Raychem Circuit Protection Division of Tyco Electronics launched PESD devices. The device has extremely low capacitance, extremely low leakage current, fast and effective ESD protection, and is cheaper than low-capacitance silicon devices. Therefore, under high-speed data transmission conditions, PESD devices have better protection application characteristics and have been successfully applied to a variety of high-speed interface circuits such as HDMI 1.3 and USB2.0.

For good ESD protection, ESD suppressor layout is also very important. Simply put, the ESD suppressor should be placed directly behind the connector. The ESD suppressor should be the first board-level component to encounter ESD transients. Then, any chips that need protection should be as far away from the ESD suppressor as practical to reduce the ESD stress on the IC.

The relative priority of PESD device mounting locations, from highest to lowest, is as follows: located inside a connector that serves as an entry point into the system shield (chassis); placed where the circuit board traces interact with the connector pins; placed just behind the connector on the board; located on a stable and unprotected transmission line that can effectively couple to the I/O lines; located before a series resistive element on the data transmission line; located before a branch point on the data transmission line; and close to the IC.

Another layout issue that needs to be considered is the distance from the PESD to the protected IC, and the choice of coupling resistors. The distance from the PESD to the protected IC should be minimized. The IC to be protected usually has its own ESD protection, but this is only device-level protection and has poor consistency, requiring PESD devices to assist/couple to the device/system-level ESD protection. As the distance from the transmission line increases, the ESD suppressor becomes increasingly "isolated" from the signal line it protects. The inductance associated with the circuit board traces, as well as any package parasitic inductance, will add impedance to the protection circuit, becoming the coupling impedance between the PESD voltage suppressor and the IC. Because the IC chip will be subjected to the sum of the voltage across the suppressor and the coupling impedance, the ideal design should make the PESD bear as much stress as possible while ensuring that there are no missing "dead corners" between the two levels of protection. Of course, the routing of multiple high-frequency signal channels has its own requirements, both to ensure transmission and to avoid mutual influence.

Finally, the chassis (frame) ground should be the ESD reference, not the signal (digital) ground, in order to shield ESD from the signal environment. By using the chassis ground as the reference, the ESD TVS protection device can be protected from unwanted noise effects (such as ground bounce), thereby maintaining a "clean" signal (data) environment as much as possible.

The figure shows a typical application diagram of PESD devices protecting HDMI interface circuits.

Typical ESD protection design schematic in HDMI interface circuit